As I understand it (not an expert but interested amateur) the sodium vapor method of a matte was a cheaper version of mattes movies used to use. To make a matte

As I understand it (not an expert but interested amateur) the sodium vapor method of a matte was a cheaper version of mattes movies used to use. To make a matte they'd film the actors over a black background with normal lighting for the actors. The black was velvet or batting that didn't reflect any of the foreground lights. They'd take that film and do a bunch of exposures that would eventually create a white silhouette that would preserve a decent amount of fine detail from the foreground. That matte would be used in an optical printer to double expose the film, once with the background plate then with the foreground plate. While effective this was really expensive and time consuming. The sodium vapor light technique allowed for a very easily generated matte with a high level of detail. It however required specific lights and a lot of control over spillage from the background lighting into the foreground. It was also difficult to film from multiple angles since the lighting would need to be adjusted when the camera was repositioned. With those limitations in mind it was much cheaper and less time consuming than making mattes with multiple exposures and such. Cheaper effect shots meant more effect shots for the same size budget. With a movie like Mary Poppins that wanted a ton of effects shots the traditional method of making mattes would have been cost prohibitive. Chroma keys came about because they were much cheaper than the multiple exposure method of doing mattes. The back and foreground could be lit together and the matte could be knocked out relatively easily by applying a chroma filter when making the work print. The mattes weren't as exact and had their own limitations but they were relatively inexpensive. For video (as opposed to film) chroma keys could be done electrically by filtering and mixing the video signal. Digital mattes have a lot of flexibility. As shown in the video digital editing software has lots of toggles to help make chroma mattes look good. It's a very manual process and hard to really get right. Also as shown in the video there's some things that just do not work really well with chroma keying like translucent materials and anything with high index of refraction. They're typically a good balance of cheap and effective with a lot of known techniques to make look good.